Temperature-sensitive binding of alpha-glucans by Bacillus stearothermophilus

Abstract

Bacillus stearothermophilus was found to bind strongly to starch and related alpha-glucans at 25 degrees C but not at 55 degrees C. The binding at the lower temperature could be assayed either by binding of fluorescein-labeled amylopectin to washed cell suspensions or through the reversible retention of bacteria by affinity chromatography in matrices containing immobilized starch. The bacteria exhibited amylopectin-dependent agglutination. The binding and agglutination were highest in bacteria grown on substrates containing alpha-1,4-glucosidic linkages such as maltose or dextrins. The binding affinity of cells was highest for maltohexaose, lower for maltose, and low or undetectable for glucose, isomaltose, cellobiose, or lactose. The reduced binding at the higher temperature was due to the rapid breakdown of the alpha-glucosides. The bacteria exhibited an extracellular alpha-amylase activity as well as a cell-associated alpha-glucosidase with high activity at 55 degrees C but undetectable activity at 25 degrees C. The inducibility, specificity, and protease sensitivity of the thermophilic alpha-glucosidase in whole cells were similar to those of the binding activity assayed at the lower temperature. Further evidence linking the binding and alpha-glucosidase activities came from a mutant, selected through affinity chromatography, which was reduced in starch binding at room temperature and also reduced in membrane-associated alpha-glucosidase activity at 55 degrees C. These results suggest a novel survival mechanism whereby a bacterium attaches to a macromolecular substrate under nonoptimal growth conditions for possible utilization upon a shift to more favorable conditions.